It's a grading system, based on three grading criteria, each of which can score up to an 'A':
Game success among critics/reviewers
Given the major reviewers comments on "Borderlands: The Pre-Sequel", the fact that it's "Yet Another First Person Shoot", and the company being unable to afford to remain in business, rather than "AAA Studio", it'd probably be better to describe 2K as a "BCF" studio.
They definitely get an F on their financial success, and YAFPS is hardly innovative game play, so they get a grudging C there, and the reviews at the top sites give them generally in the neighborhood of an 80% approval by reviewers (only GameStop rates them higher than 80%), so that's a B.
I really hate that people hype studios themselves as "AAA", as if that means they are going to get A's in all three categories, just because of who they are, or because of the marketing hype behind their games contributing to a likelihood of good reviews or financial success.
In reality, you are only as good as your last release in all three categories. 2K blew it in at least two of the categories, and turned in B grade work in the third, so it's no surprise they failed.